Control rods in a nuclear reactor perform the functions of power distribution, shaping and reactivity control. This is accomplished generally by providing a plurality of control rods containing material for neutron absorption and manipulating the control rods within the reactor core and between the fuel bundle assemblies. Typically, each control rod employed in a boiling water reactor is provided in a cruciform cross-sectional shape and extends in complementary cruciform-shaped interstices between the fuel bundles.
Generally, control rod guide tubes are located inside the pressure vessel and extend from the top of the control rod drive housing through the core support. Typically, each tube is designed as a lateral guide for a control rod and as a vertical support for a four-lobed fuel support piece and the fuel assemblies surrounding the control rod. The bottom of each guide tube is supported by the control rod drive housing which, in turn, transmits the weight of the guide tube, fuel support piece and fuel assemblies to the reactor vessel bottom head. Linear hydraulic systems are employed to drive the control rods. These drive systems, however, involve complicated control systems, as well as space for accumulators, valves, piping and controls.